Taseko Mines Ltd. has a blunt message for Natural Resources Canada: you got it wrong.

But it is unclear what the federal government thinks of that message, or how it will impact Taseko’s efforts to build the New Prosperity mine in British Columbia.

Taseko’s statement came less than a week after a Federal Review Panel issued a damning report on New Prosperity. According to the panel, the proposed mine could create “significant adverse environmental effects” on local fish habitats and First Nations interests because of potential seepage from a tailings facility into nearby Fish Lake.

The panel report is important, as the government will review it before ruling on whether New Prosperity should go ahead. A highly negative report can swing Ottawa’s decision.

According to Taseko, the report is badly flawed.

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The company said it appears that Natural Resources Canada (NRC), a federal agency, studied the wrong tailings design when it evaluated New Prosperity. As a result, the review panel determined that the US$1.1-billion mine would have a seepage rate that is many times higher than Taseko proposed.

“It looks like they reviewed their own plan instead of Taseko’s plan,” Taseko vice-president Brian Battison said in an interview. “And when they reviewed their own plan, they found that it had significant adverse environmental effects.”

He said the NRC evaluated a tailings design that did not include a liner at the bottom of the basin. That liner, which is part of Taseko’s plan, is designed specifically to prevent seepage.

“I don’t know whether they missed it, chose not to believe it, or chose to go with their own design,” Mr. Battison added.

A spokesman from the Canadian Environmental Assessment Agency declined comment on the panel report.

When the report was released late last week, Taseko was astounded at the findings. The company claims its tailings storage proposal uses the same seepage control methods as Mount Polley and Mount Milligan, two existing mines in British Columbia. In fact, the consultant — Knight Piesold — that designed Mount Milligan’s tailings facility also worked on New Prosperity. Knight Piesold recently won an award of merit for its work at Mount Milligan.

The federal government is expected to make a decision on New Prosperity within four months. No matter what it decides, the ruling will be controversial given the turbulent history of this project.

Ottawa rejected Taseko’s first effort to develop this mine in 2010, saying the environmental damage would be too great. Taseko planned to drain Fish Lake, which upset local First Nations leaders. The federal government rejected the proposal even though the British Columbia government approved it.

Taseko then came up with a new mine design that would save Fish Lake for an additional cost of US$300-million. It is up to Leona Aglukkaq, the federal environment minister, to decide whether this version should go ahead.

“The government has this flawed finding that they need to deal with. And the minister will need to take this into consideration,” Mr. Battison said.